Rod-type polysilicon having improved breaking properties

ABSTRACT

Rod-type, polycrystalline silicon having a rod diameter of &gt;100 mm are obtained by deposition of silicon-containing gas according to the Siemens method, wherein the Si rods are brought into contact with hydrogen at the end of the deposition process during cooling in the reactor, and the cooled Si rods obtained have in perpendicular cross section cracks and/or radial stresses having a defined size.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/878,085, filed Sep. 9, 2010 (pending), which claims priority toGerman Patent Application No. 10 2009 044 991.4 filed Sep. 24, 2009, allof which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to rod-type polysilicon having improved breakingproperties, obtained according to the Siemens deposition method.

2. Background Art

Polycrystalline silicon (polysilicon) serves as a starting material forproducing monocrystalline silicon for semiconductors according to theCzochralski or floating zone method, and also for producing mono- ormulticrystalline silicon according to various pulling and castingmethods for the production of solar cells for photovoltaics.

In this case, the polysilicon is principally produced by depositionusing trichlorosilane according to the so-called Siemens method. In thismethod, in a bell-shaped reactor, the so-called “Siemens reactor”, thinfilament rods composed of silicon are heated by direct passage ofcurrent and brought into contact with a reaction gas composed ofhydrogen and one or more silicon-containing components. In this case, apreferred raw material is trichlorosilane (SiHCl₃) or a mixture thereofwith dichlorosilane (SiH₂Cl₂).

The filament rods are inserted perpendicularly in electrodes which aresituated on the base of the reactor and via which the connection to thepower supply is effected. High-purity polysilicon deposits on the heatedfilament rods and on the horizontal bridge that respectively connectstwo rods, as a result of which the rod diameter increases with time.

After the desired diameter has been attained, the further addition ofSi-containing components ceases. The reactor is subsequently purged inorder to remove all gaseous reaction products and the residues of theSi-containing components. The purging is generally effected either usinghydrogen or using an inert gas such as nitrogen or argon. After purging,the inflow of the purging gas is stopped and the supply of energy isreduced, either abruptly or with a specific ramp to zero, and theresulting Si rods cool then to the ambient temperature.

After the rods have cooled and if the purging was not effected usinginert gas, hydrogen in the reactor bell is replaced by an inert gas, thedeposition reactor is opened and the carrier bodies with the polysiliconrods are removed from the reactor.

For various uses of the polysilicon rods it is then necessary to breakthe rods into small pieces in a subsequent step. Si rods producedaccording to the conventional Siemens method are very hard and thereforerequire high forces to break them apart. This has the effect that,during the corresponding breaking method, Si fragments on the surfaceare significantly contaminated with material of breaking tools.Moreover, after a breaking, the intention is for as many Si fragments aspossible to be in a preselected size range. The process of breaking Sirods according to the prior art gives rise to many fragments which donot correspond to the desired size and, consequently, have to be sold atsignificantly reduced prices.

EP 0329163 describes a method for improving the breaking properties ofpolysilicon wherein the Si rods, after deposition, are once again heatedand quenched or subjected to a compression wave in an aftertreatment.However, this method is associated with very high technical complexity,high costs, and risk of contamination.

During the production of thick polycrystalline Si rods it can relativelyoften be observed that they tilt from the mounts and fall over duringthe cooling phase in the reactor after the deposition. This phenomenondelays the production process considerably, since the rods that havefallen over can only be removed from the reactor with additionalcomplexity. Furthermore, this also gives rise to high financial damagesince tilted or collapsed Si rods can no longer be processed further asenvisaged. The financial damage is particularly high in the productionof polysilicon for the solar industry, because this material is normallyused without additional cleaning steps. Rods that have fallen oversubsequently have to be subjected to complex cleaning, which makes thesolar material significantly more expensive.

U.S. Pat. No. 5,593,465 discloses arranging at least one spring elementbetween the current feed and the electrode holder, the spring elementpermitting a movement of the electrode holder relative to the currentfeed and cushioning this movement. This arrangement is intended tominimize tilting and falling of the Si rods. What is disadvantageousabout this solution, however, is the high technical complexity and theconsiderable costs associated therewith.

U.S. Pat. No. 6,639,192 recommends the use of carbon electrodes having ahigh thermal conductivity in order to prevent the rods from falling overin the reactor. This solution has the serious disadvantage, however,that, owing to the high thermal conductivity of carbon electrodes, thelatter are not overgrown or are overgrown only little by the siliconduring the deposition process. This has the effect that the rods caneasily fall over as early as at the beginning of the deposition.

SUMMARY OF THE INVENTION

An object of the present invention is to provide polycrystalline siliconrods having a large diameter (>100 mm) composed of high-purity siliconfor applications in the semiconductor industry and also in the solarindustry for various pulling and casting methods for the production ofsolar cells for photovoltaics, which have an improved breaking behaviorin order to significantly reduce the associated contamination of Sifragments and to reduce the proportion of silicon fragments havingundesired sizes. A further aim of the invention is to reduce theproduction costs of the polycrystalline silicon rods by preventing therods from falling over during the cooling phase. It has surprisinglybeen found that polycrystalline silicon rods, by means of a specificwithdrawal process with addition of hydrogen at the end of thedeposition according to the Siemens method, acquire defined cracks andstresses and can thus be broken into pieces more easily in subsequentfurther processing. A further surprising advantage of the aftertreatmentaccording to the invention is the reduction of the proportion of rodswhich fall over in the reactor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The invention thus relates to rod-type, polycrystalline silicon having arod diameter of >100 mm, obtained by deposition of silicon-containinggas according to the Siemens method, wherein the Si rods are broughtinto contact with hydrogen at the end of the deposition process duringcooling in the reactor, wherein the hydrogen flow rate and/or thehydrogen pressure is selected such that the power required in the caseof the selected flow rate and/or pressure for maintaining the depositiontemperature is at least 50% of the power at the end of deposition, butnot less than 5 kW per 1 m rod length, and the cooled Si rods have inperpendicular cross section cracks and/or radial stresses having a sizeof at least 1·10⁻⁴ cm⁻¹.

The inventive rod-type, polycrystalline silicon having a rod diameterof >100 mm arises by virtue of the fact that, during the cooling of theSi rods, a high quantity of hydrogen is introduced into the reactorand/or the pressure in the reactor is raised. In this case, the hydrogenis preferred to be present during the phase of reducing the temperaturefrom the deposition temperature to the ambient temperature. Theapplication of hydrogen can also already be ended at higher temperaturesthan the ambient temperature, but it has to be maintained at least to arod temperature of 800° C.

The hydrogen flow rate and/or the pressure must be chosen such that thepower required in the case of the selected flow rate and pressure formaintaining the deposition temperature is at least 50% of the power atthe end of deposition, but not less than 5 kW per 1 m rod length.

In this case, preference is given to a pressure in the reactor ofbetween 2 and 30 bar, more preferably between 3 and 20 bar. The hydrogenflow rate is preferably between 2 and 100 Nm³/h per 1 m rod length, morepreferably between 5 and 50 Nm³/h per rod length.

This procedure has the effect that Si rods are highly strained andacquire many cracks during cooling. As a result, the Si rods can bebroken very easily in the further processing, such that the Si fragmentsare contaminated only very little by the abrasion of the breaking toolsused. A further advantage is that the proportion of lower-quality smallsilicon fragments is reduced.

If the cross sections of the Si rods are considered in a perpendiculardirection with respect to the thin rod, then at least 80% of the crosssections have cracks and/or radial stresses larger than 1·10⁻⁴ cm⁻¹.These cracks in the polycrystalline Si rods can be detected visually orby means of known methods such as, for example, sound testing, dyepenetration testing or ultrasonic technology. A method for measuringmechanical stresses in polycrystalline rods is described, by way ofexample, in U.S. Pat. No. 5,976,481.

In the case of breaking compact polycrystalline Si rods, fragmentshaving a size in the range of 50 to 100 mm are preferred for mostapplications. In the case of conventional Si rods, a maximum proportionby weight of 65% of fragments of the target size, relative to the totalquantity, is obtained by means of known breaking methods. In this case,the proportion of fragments having a size of less than 10 mm is 2.5 to5%. In contrast thereto, in the case of breaking the rod-typepolycrystalline silicon rods according to the invention, fragmentshaving a size in the range of 50 to 100 mm are increased significantlyto greater than 70%, relative to the total quantity. In this case, theproportion of small fragments below 10 mm can be reduced to less than2%. Given the quantities in industrial production that are customarynowadays, amounting to tens of thousands of tons per year, this means ahuge cost saving.

Tools composed of tungsten carbide are generally used in the breakingmethods known from the prior art. In this case, the surface of thebroken target material is contaminated with 1 to 2 ppbw of tungsten.When the material according to the invention is used, by contrast, theimportant fragments having a size of 50 to 100 mm have contamination ofless than 0.8 ppbw. When breaking tools composed of other materials areused, the contamination by the tool material is likewise significantlyreduced. It has thus been found that the average contamination isreduced by at least 20% by the use of the material according to theinvention.

It has surprisingly been observed as a further effect of thepolycrystalline Si rods according to the invention that these rods tendto fall over to a lesser extent in the reactor. The proportion of Sirods that fell over was able to be reduced by more than 50% incomparison with the prior art. By virtue of the stresses and cracksaccording to the invention in the Si rods, although the latter can bebroken more easily by means of breaking tools, at the same time theyhave a higher loading stability with respect to falling over whilestanding in the reactor.

The invention will be explained in greater detail on the basis of thefollowing examples.

The examples were carried out in a Siemens reactor with 8 rods. The thinrods used were composed of ultrapure silicon having a length of 2 m andhad a diameter of 5 mm. A mixture of hydrogen and trichlorosilane wasused for deposition. The temperature of the rods was 1000° C. during theentire deposition time. The pressure in the reactor was 3 bar. Thedeposition proceeded until the rods attained a diameter of 160 mm. Thepower required at the end of the deposition was approximately 25 kW per1 m rod length.

Comparative Example

After the rods had attained a diameter of 160 mm, the supply oftrichlorosilane was ended. Afterward, the reactor was purged using purehydrogen for 1 hour. The supply of hydrogen was then ended. In thiscase, the required power was 5 kW per 1 m rod length. The temperature ofthe rods was then reduced from 1000° C. to 500° C. within one hour andthe power supply was subsequently switched off. After cooling of therods and replacement of the gas in the reaction bell by nitrogen, thedeposition reactor was opened and the carrier bodies were removed fromthe reactor. In a subsequent step, the rods were broken in a targetedmanner into pieces having a size of 50 to 100 mm using hammers composedof tungsten carbide. In total, 100 batches were deposited by means ofthis method. 10% of the rods fell over during cooling and were unusablewithout subsequent cleaning. A plurality of cross sections were analyzedfor each rod. Approximately 30% of the analyzed cross sections hadcracks and/or stresses of larger than 1·10⁻⁴ cm⁻¹. When the rods werebroken, approximately 3.5% of the fragments obtained had a size of lessthan 10 mm. The preferred Si fragments having the target size of 50 to100 mm were obtained on average in a proportion of 61% and had onaverage tungsten contamination of 3.4 ppbw.

Example 1

After the rods had attained a diameter of 160 mm, the supply oftrichlorosilane was ended. The reactor was subsequently purged usingpure hydrogen for 1 hour. Afterward, the hydrogen pressure in theinstallation was raised to 10 bar and the hydrogen flow rate was raisedto 200 Nm³/h. In this case, the required power was 15 kW per 1 m rodlength. The temperature of the rods was then reduced from 1000° C. to500° C. within one hour and the power supply was subsequently switchedoff. After cooling of the rods and replacement of the gas in thereaction bell by nitrogen, the deposition reactor was opened and thecarrier bodies were removed from the reactor. In a subsequent step, therods were broken in a targeted manner into pieces having a size of 50 to100 mm using hammers composed of tungsten carbide. In total, 200 batcheswere deposited by means of this method according to the invention. Incontrast to the prior art, only 3% of the rods fell over during cooling.A plurality of cross sections were analyzed for each rod. Approximately92% of the analyzed cross sections had cracks and/or stresses largerthan 1·10⁻⁴ cm⁻¹. When the rods were broken, approximately 1.5% of thefragments were obtained with a size of less than 10 mm. The preferred Sifragments having the target size of 50 to 100 mm were obtained onaverage in a proportion of 77% and had on average tungsten contaminationof 0.5 ppbw.

Example 2

After the rods had attained a diameter of 160 mm, the supply oftrichlorosilane was ended. The reactor was subsequently purged usingpure hydrogen for 1 hour. The hydrogen flow rate was then raised to 200Nm³/h. The pressure in the installation was set to be equal to theambient pressure (approximately 1 bar). In this case, the required powerwas 13 kW per 1 m rod length. The temperature of the rods was thenreduced from 1000° C. to 500° C. within one hour and the power supplywas subsequently switched off. After cooling of the rods and replacementof the gas in the reaction bell by nitrogen, the deposition reactor wasopened and the carrier bodies were removed from the reactor. In asubsequent step, the rods were broken in a targeted manner into pieceshaving a size of 50 to 100 mm using hammers composed of tungstencarbide. In total, 100 batches were deposited by means of this methodaccording to the invention. In contrast to the prior art, 5% of the rodsfell over during cooling. A plurality of cross sections were analyzedfor each rod. Approximately 85% of the analyzed cross sections hadcracks and/or stresses larger than 1·10⁻⁴ cm⁻¹. When the rods werebroken approximately 1.8% of the fragments were obtained with a size ofless than 10 mm. The preferred Si fragments having the target size of 50to 100 mm were obtained on average in a proportion of 73% and had onaverage tungsten contamination of 0.7 ppbw.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. A method for producing polycrystalline siliconrods having a rod diameter >100 mm, comprising: a) supplying asilicon-containing gas to a reactor containing silicon thin rods heatedby electric power to a deposition temperature at which thesilicon-containing gas deposits polycrystalline silicon onto the thinrods according to the Siemens process to form polycrystalline rods,increasing the diameter of the polycrystalline rods, and continuingsupplying the silicon-containing gas at the deposition temperature untilthe diameter of the polycrystalline rods is >100 mm; b) ceasing thesupply of silicon-containing gas and introducing hydrogen gas into thereactor at the deposition temperature, the hydrogen gas being introducedinto the reactor at a pressure and at a flow rate such that thedeposition temperature is maintained at a reduced power input which is≧5 kW/m of rod length, and not less than 50% of the electric power ofstep a); and c) reducing the electric power and cooling the rods in thepresence of hydrogen gas; and d) removing the rods from the reactor,wherein the rods removed from the reactor have cracks and/or radialstresses in an amount not less than 1·10⁻⁴ cm⁻¹ in 80% or more of crosssections of the polycrystalline rod.
 2. The method of claim 1, whereinin step c), the polycrystalline rods are cooled down to a temperaturewhich is 800° C. or less, and the electric power is shut off.
 3. Themethod of claim 2, wherein the hydrogen flow rate in step b) is between2 and 100 Nm³/h per 1 m rod length.
 4. The method of claim 1, wherein instep c), the polycrystalline rods are cooled down to a temperature whichis 500° C. or less, and the electric power is shut off.
 5. The method ofclaim 4, wherein the hydrogen flow rate in step b) is between 2 and 100Nm³/h per 1 m rod length.
 6. The method of claim 1, wherein the hydrogenpressure in the reactor in step b) is between 2 and 30 bar.
 7. Themethod of claim 6, wherein the hydrogen flow rate in step b) is between2 and 200 Nm³/h per 1 m rod length.
 8. The method of claim 1, whereinthe hydrogen pressure in the reactor in step b) is between 3 and 20 bar.9. The method of claim 8, wherein the hydrogen flow rate in step b) isbetween 2 and 100 Nm³/h per 1 m rod length.
 10. The method of claim 1,wherein the hydrogen flow rate in step b) is between 2 and 200 Nm³/h per1 m rod length.
 11. The method of claim 1, wherein the hydrogen flowrate in step b) is between 5 and 100 Nm³/h per 1 m rod length.
 12. Themethod of claim 11, wherein the hydrogen flow rate in step b) is between2 and 100 Nm³/h per 1 m rod length.
 13. The method of claim 1, whereinthe polycrystalline rods are cooled to 500° C. from a depositiontemperature of 1000° C. within an hour.
 14. The method of claim 1,wherein the deposition temperature is from 900° C. to 1000° C.